Optoelectronic devices rely on the optical and electronic properties of materials to either produce or detect electromagnetic radiation electronically or to generate electricity from ambient electromagnetic radiation.
Photosensitive optoelectronic devices convert electromagnetic radiation into an electrical signal or electricity. Solar cells, also called photovoltaic (“PV”) devices, are a type of photosensitive optoelectronic device that is specifically used to generate electrical power. Photoconductor cells are a type of photosensitive optoelectronic device that are used in conjunction with signal detection circuitry which monitors the resistance of the device to detect changes due to absorbed light. Photodetectors, which may receive an applied bias voltage, are a type of photosensitive optoelectronic device that are used in conjunction with current detecting circuits which measures the current generated when the photodetector is exposed to electromagnetic radiation.
These three classes of photosensitive optoelectronic devices may be distinguished according to whether a rectifying junction is present and also according to whether the device is operated with an external applied voltage, also known as a bias or bias voltage. A photoconductor cell does not have a rectifying junction and is normally operated with a bias. A PV device has at least one rectifying junction and is operated with no bias. A photodetector has at least one rectifying junction and is usually but not always operated with a bias.
As used herein, the term “rectifying” denotes, inter alia, that an interface has an asymmetric conduction characteristic, i.e., the interface supports electronic charge transport preferably in one direction. The term “semiconductor” denotes materials which can conduct electricity when charge carriers are induced by thermal or electromagnetic excitation. The term “photoconductive” generally relates to the process in which electromagnetic radiant energy is absorbed and thereby converted to excitation energy of electric charge carriers so that the carriers can conduct (i.e., transport) electric charge in a material. The term “photoconductive material” refers to semiconductor materials which are utilized for their property of absorbing electromagnetic radiation to generate electric charge carriers.
When electromagnetic radiation of an appropriate energy is incident upon an organic semiconductor material, a photon can be absorbed to produce an excited molecular state. In organic photoconductive materials, the generated molecular state is generally believed to be an “exciton,” i.e., an electron-hole pair in a bound state which is transported as a quasi-particle. An exciton can have an appreciable life-time before geminate recombination (“quenching”), which refers to the original electron and hole recombining with each other (as opposed to recombination with holes or electrons from other pairs). To produce a photocurrent, the electron-hole forming the exciton are typically separated at a rectifying junction.
In the case of photosensitive devices, the rectifying junction is referred to as a photovoltaic heterojunction. Types of organic photovoltaic heterojunctions include a donor-acceptor heterojunction formed at an interface of a donor material and an acceptor material, and a Schottky-barrier heterojunction formed at the interface of a photoconductive material and a metal.
FIG. 1 is an energy-level diagram illustrating an example donor-acceptor heterojunction. In the context of organic materials, the terms “donor” and “acceptor” refer to the relative positions of the Highest Occupied Molecular Orbital (“HOMO”) and Lowest Unoccupied Molecular Orbital (“LUMO”) energy levels of two contacting but different organic materials. If the LUMO energy level of one material in contact with another is lower, then that material is an acceptor. Otherwise it is a donor. It is energetically favorable, in the absence of an external bias, for electrons at a donor-acceptor junction to move into the acceptor material.
As used herein, a first HOMO or LUMO energy level is “greater than” or “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level 10. A higher HOMO energy level corresponds to an ionization potential (“IP”) having a smaller absolute energy relative to a vacuum level. Similarly, a higher LUMO energy level corresponds to an electron affinity (“EA”) having a smaller absolute energy relative to vacuum level. On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material.
After absorption of a photon 6 in the donor 152 or the acceptor 154 creates an exciton 8, the exciton 8 dissociates at the rectifying interface. The donor 152 transports the hole (open circle) and the acceptor 154 transports the electron (dark circle).
A significant property in organic semiconductors is carrier mobility. Mobility measures the ease with which a charge carrier can move through a conducting material in response to an electric field. In the context of organic photosensitive devices, a material that conducts preferentially by electrons due to a high electron mobility may be referred to as an electron transport material. A material that conducts preferentially by holes due to a high hole mobility may be referred to as a hole transport material. A layer that conducts preferentially by electrons, due to mobility and/or position in the device, may be referred to as an electron transport layer (“ETL”). A layer that conducts preferentially by holes, due to mobility and/or position in the device, may be referred to as a hole transport layer (“HTL”). Preferably, but not necessarily, an acceptor material is an electron transport material and a donor material is a hole transport material.
How to pair two organic photoconductive materials to serve as a donor and an acceptor in a photovoltaic heterojunction based upon carrier mobilities and relative HOMO and LUMO levels is well known in the art, and is not addressed here.
One common feature of bulk semiconductors, as well as insulators, is a “band gap.” The band gap is the energy difference between the highest energy level filled with electrons and the lowest energy level that is empty. In an inorganic semiconductor or inorganic insulator, this energy difference is the difference between the valence band edge (top of the valence band) and the conduction band edge (bottom of the conduction band). In an organic semiconductor or organic insulator, this energy difference is the difference between the HOMO and the LUMO. The band gap of a pure material is devoid of energy states where electrons and holes can exist. The only available carriers for conduction are the electrons and holes which have enough energy to be excited across the band gap. In general, semiconductors have a relatively small band gap in comparison to insulators.
In terms of an energy band model for organic semiconductors, only electrons on the LUMO side of the band gap are charge carriers, and only holes on the HOMO side of the band gap are charge carriers.
Additional background explanation and description of the state of the art for organic photosensitive devices, including their general construction, characteristics, materials, and features, can be found in U.S. Pat. No. 6,657,378 to Forrest et al., U.S. Pat. No. 6,580,027 to Forrest et al., and U.S. Pat. No. 6,352,777 to Bulovic et al., the disclosures of which are incorporated herein by reference.
The performances of small molecular solar cells are determined by studying their characteristic IV responses under dark conditions and under illumination. The power conversion efficiency, ηp, is dependent on the open circuit voltage (Voc), the short -circuit current density (Jsc), and the fill factor (FF) via [Ref. 1]:
                              η          p                =                              (                                          J                sc                            ×                              V                oc                            ×              FF                        )                    /                      P            o                                              (        1        )            where Po is the incident optical power. Here, FF depends on the series resistance and is typically between 0.5 and 0.65 for high performance small molecular weight organic photovoltaics. The maximum Jsc is defined by the overlap between the absorption of the organics, the solar spectrum and the extinction coefficients and thicknesses of the absorbing layers and other factors. However, the photocurrent is highly dependent on the charge transport properties of the materials, since resistivity to charge flow represents a significant challenge to cell performance [Ref. 2]. Another very important parameter to be considered when referring to cell performance is the exciton diffusion length. The exciton diffusion length of a material represents the distance that an exciton can travel prior to recombination. Accordingly, in order to achieve a high percentage of charge carriers relative to the number of excitons created by absorbed photons the exciton is preferably formed within about LD of a Heterojunction. The exciton diffusion length, LD, is related to the exciton diffusion coefficient, D, and the exciton lifetime, τ, by the expression: LD=√{square root over (Dτ)}. The exciton diffusion length is generally short for organic semiconductors relative to the optical absorption length LA, hence limiting the thickness of the organic layer to be used due to the relatively low ability of the excitons to reach the Donor-Acceptor interface for charge separation. This effect not only restrains the amount of absorbing material but also creates a resistive pathway for separated charge that is undesirable for efficient light conversion [Ref. 1].
The origin of Voc, in organic solar cells is not well understood [Refs. 3,4]. Some people suggest that it is mainly dependent on the energy difference between the lowest unoccupied molecular orbital (LUMO) of the acceptor-like material and the highest occupied molecular orbital (HOMO) of the donor-like material at the heterointerface in a bilayer cell (referred to as the interface gap, Ig) [Ref. 5]. However others have observed no evident relation between this Ig and the Voc observed and propose that this voltage is controlled by a chemical potential gradient that would depend on the carrier mobility [Ref. 6]. Yet, it is clear that the Voc does not reflect the total energy of the photons absorbed and that energy must be lost during the power conversion process. These losses have not been accounted for so far and much care must be taken when assessing the foundations of the open-circuit voltage.
Organic photovoltaic (OPV) cells have great promise to become a viable alternative to the existing solar cell technologies, dominated by silicon based devices. However, their efficiencies are currently too low to compete effectively with Si based devices. The record efficiencies for laboratory based OPV cells is 5.7%, [Ref. 7] which is roughly half the efficiency of commercial amorphous silicon based PV cells. An inherent limitation of the current OPV systems is related to light collection efficiency, which is controlled by the excitons diffusion length. New device architectures have been developed to compensate for the short diffusion lengths, but there has not been a detailed study of applicable materials, focused on solving the problem by extending the diffusion length. [Refs. 8,9]
The basic mechanism of photocurrent generation in organic photovoltaic cells (OPVs) can be illustrated with two organic materials, one a net electron donor (D) and the other an acceptor (A). The process can be broken down into four sequential steps (FIG. 2). The first step of the process is light absorption, leading to exciton formation, with an efficiency given by ηA. In order to cover a large fraction of the solar spectrum, the donor and acceptor materials chosen for OPVs must have broad absorbance lines and high extinction coefficients, giving a high optical density for thin films.
Once formed, the exciton then migrates to the D/A interface, or alternatively decays to the ground state via radiative or nonradiative processes (kdecay). The optimal thickness is determined by the exciton diffusion length, since excitons generated beyond this distance decay faster then they migrate to the D/A interface. The efficiency of exciton diffusion (ηED) is related to the relative magnitudes of the diffusion rate (kdiff) and the decay rate (kdecay). The majority of the OPV materials that have been examined have short exciton diffusion lengths. For example, CuPC and C60, common donor and acceptor materials, have exciton diffusion lengths of 8-10 and 40 nm, respectively. The short exciton diffusion lengths seen for these materials forces the D and A films to be kept thin, limiting the optical density in OPVs, and thus the amount of light that can be efficiently collected and the efficiency of OPVs.
At the D/A interface, the exciton undergoes a charge transfer reaction (efficiency=ηCT), forming a hole and electron in the D and A layers, respectively. The driving force for charge transfer is the energy offset between the donor and acceptor orbitals. After the hole and electron are generated, they are conducted through the D and A materials and extracted by the electrodes (charge collection). A high carrier mobility is critical for an efficient OPV. A low mobility (high resistance) material will leave charges trapped near the D/A interface, promoting back electron transfer (recombination of the hole and electron at the interface). This leads to a decreased photocurrent and a marked decrease in the fill factor, both contributing to lower the power available for the device.
A number of different approaches have been explored to increase the optical densities of OPVs, without decreasing exciton collection at the D/A interface. One beneficial approach is to stack the OPVs, making a tandem cell [Refs. 9,10], for example, see FIG. 3A. Tandem OPVs have been shown to give up to two times the power output of a single OPV of the same structure. [Ref. 9] An alternate approach has been used broadly in polymer based OPVs, which involves the use of a bulk heterojunction. [Refs. 11,12] If the D and A materials are mixed, they tend to phase separate into microdomains of pure D and A, which provide the respective charge conducting channels. This bulk heterojunction material can be made thick enough to absorb the majority of the incident light, without forming excitons that are too far from the D/A interface to be collected. (FIGS. 3A-B)
The basic strategy for attacking the short exciton diffusion lengths in OPV materials in both tandem and bulk heterojunction devices involve increasing light absorbance, without trying to deal with the short exciton diffusion length directly. These approaches involve device structure changes and not new materials design. The problem with this approach is that it does not address the underlying problem, which is the short exciton diffusion length of these organic semiconductors.
The exciton diffusion length, LD, is related to the exciton diffusion coefficient, D, and the exciton lifetime, τ, by the expression: LD=√{square root over (Dτ)}. The highest exciton diffusion coefficients will be observed for single crystalline thin films. The amorphous glasses used in typical OPV cells tend to have lower diffusion coefficients, due to exciton scattering at defect sites. The exciton formed on light absorption is in a singlet configuration. The radiative lifetime of this exciton is typically 10 nsec or less. The measured lifetimes for singlet excitons in neat thin films are markedly lower than their radiative lifetime, due to excimer formation, quenching at defects and unimolecular nonradiative decay, principally intersystem crossing to the triplet exciton. The exciton diffusion coefficient for a singlet exciton can be rather high, since Forster energy transfer processes are operative, allowing hops of several nm at a time. Triplet excitons have lifetimes that are markedly longer than their singlet counterparts. While an organic molecule can have a singlet lifetime in the nsec range, the triplet lifetime of the same material can be as high as minutes to hours in a dilute sample. Based on the LD=√{square root over (Dτ)} relationship, it would be expected to see long diffusion lengths for triplet excitons. While triplet exciton diffusion lengths can be reasonably long, for example C60 gives an LD of 40 nm for an amorphous thin film, they are not as long as one would have guessed from the triplet lifetime. There are several reasons for this. The first reason is that the lifetime in a neat thin film is markedly less than that in a dilute matrix, where it is often measured. Quenching, excimer formation and T-T annihilation decrease triplet lifetimes by several orders of magnitude. Also, the diffusion coefficients for triplets are markedly lower than those for singlets. In contrast to singlets, Forster energy transfer processes are not very efficient, due largely to the low oscillator strengths for triplet absorption bands. Thus, triplet exciton diffusion occurs mainly through nearest neighbor hopping, via a Dexter mechanism. While the triplet lifetimes for common organic molecules can be very long, common OPV materials have triplet lifetimes in the microsecond regime. This coupled with the low D values give triplet based materials good but not exceptional LD values.
If one could increase the exciton diffusion length for OPV materials, devices could be fabricated with thick, optically dense D and A layers. Thus, there is a need to provide organic photosensitive optoelectronic devices having improved properties via new materials design, which addresses the issue of the exciton diffusion length.